In the life cycle of HIV, the Rev protein regulates the temporal switch from the early regulatory to the late lytic phase by binding to a highly structured RRE (Rev Responsive Element) RNA. Our studies of Rev and RRE RNA were targeted: i) to analyze the various functional motifs of the Rev protein; and ii) to characterize the function of the putative cellular factors that may bind RRE RNA, Rev, or both. Since last year, we have further characterized the biochemical properties of the cellular ds RNA binding protein, TRBP that bound to the RRE# RNA. TRBP was a potent inhibitor of the interferon induced PKR kinase in vitro and in vivo. Both PKR and TRBP were capable of dimerizing and the PKR inhibition by TRBP was mediated by heterodimerization with PKR. We have extended our studies on the effect of HIV-1 NEF protein on T lymphocyte CD4 receptor. We demonstrated that Nef downregulates CD4 expression through a bi-modal mechanism through endocytosis of cell surface receptor and repression of biosynthesis and transport of nascent CD4. Using the yeast two hybrid system of genetic screening, we identified two HeLa cell proteins that interact with Nef. One of these, RKA 29 is a polypeptide of 255 amino acids. The polypeptide has a basic N terminal end and a highly acidic C terminus that is critical for HIV Nef binding. RK29 is a nuclear protein and associates with the unmyristylated Nef protein in HeLa cells. The acidic C- terminal portion of RK29 has homologies with transcription factors suggesting that it is itself a transcription factor and may represent a cellular cofactor for Nef effects on HIV or non HIV transcription.